The present invention relates generally to an anti-skid brake control system which can optimize braking characteristics. More particularly, the invention relates to an anti-skid brake control system featuring variable wheel slippage criteria for releasing braking force depending upon lateral force exerted on the vehicle. Further specifically, the invention relates to a process for performing anti-skid automotive brake control optimizing not only the vehicular braking characteristics but also the vehicle cornering stability.
As is well known, optimum braking characteristics are obtained when braking pressure or force can be so adjusted that the peripheral speed of the wheels during braking is held to a given ratio, e.g. about 80% to 85%, of the vehicle speed. This practice is believed to be particularly effective when road conditions and other factors are taken into consideration. On the other hand, if wheel speed is held to a ratio to the vehicle speed higher than the above-mentioned optimal ratio, e.g., 80% to 85%, braking distance may be prolonged due to lack of braking pressure.
On the other hand, if the braking pressure is so adjusted as to hold the wheel speed to a ratio with respect the vehicle speed less than the aforementioned optimal ratio, the vehicle wheels may lock and skid, resulting in an unnecessarily long braking distance due to reduced traction. In practice, it is very difficult to precisely adjust the braking pressure so that the wheel speed is held to the given optimal ratio to the vehicle speed.
In the practical anti-skid brake control operation, braking pressure in one or more wheel cylinders is adjusted by cyclically increasing and decreasing the braking pressure in the wheel cylinder.
The anti-skid control system generally decreases braking pressure when the wheel deceleration value becomes less than a given deceleration threshold, which is so chosen as to prevent the wheel from skidding, and increases braking pressure when the wheel acceleration value is greater than a given acceleration threshold. In this conventional anti-skid brake control procedure, wheel speed does not remain in an optimal relationship to the vehicle speed for a satisfactorily long period of time. In order to improve performance, U.S. Pat. No. 3,637,264, issued on Jan. 25, 1972 to Leiber et al discloses an Antilocking Control for Pressure Actuated Brakes. The pressure of the brake-actuating fluid in an antilocking brake control system is varied by pulsing the control valve or valves for durations which are varied to be greater or lesser than the period of that limiting frequency above which the brake system cannot respond.
In the former case, a rapid increase in fluid pressure or a rapid decrease in fluid pressure occurs, whereas in the latter case, a less rapid average or net increase or decrease occurs in the fluid pressure to which the brake system responds. These conditions are controlled in dependence on the rotational behavior of the vehicle wheel or wheels and more especially in dependence or predetermined changes in angular velocity of the wheel.
Moreover, either variation in pulse duration at a fixed frequency or variation in frequency at a fixed pulse duration may be effected during high-frequency pulsing so as further to alter the net increase or decrease in fluid pressure. This further alternation is effected as a function of time from the beginning of the high-frequency pulsing.
In addition, Published Japanese Patent Application (Tokkai) Showa No. 51-89096, published on Aug. 4, 1976 discloses a system similar to the above. The fluid pressure in the wheel cylinder is increased in a stepwise manner. Duration of increase of the fluid pressure is adjusted in accordance with the rate of increase of the fluid pressure in one or more preceding steps.
In addition, a vehicular cornering force tends to decrease as wheel slippage increases. Therefore, when the anti-skid control is active to maintain the wheel slippage at the optimal level, i.e. 10 to 20%, cornering force of the vehicle tends to be reduced than when there is no wheel slippage condition. This result in unstability of the vehicle behavious with respect to the transverse force possibly exerts the vehicle during cornering. Therefore, it has been conventionally considered that it is not possible to achieve both of satisfactory level of bracking characteristics and sufficient cornering stability.